common: add missing header guards to gl_canvas.h
common: add missing header guards to gl_canvas.h

SPICE/spice-common,Fantu/spice-common,fgouget/spice-common,fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,elmarco/spice-common,Fantu/spice-common,SPICE/spice-common,fgouget/spice-common,fgouget/spice-common,elmarco/spice-common,Fantu/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,SPICE/spice-common,elmarco/spice-common